1. Field of the Invention
The present invention relates to a sensing device for detecting a change in an applied magnetic field, and more particularly, to a sensing device which is particularly suitable for detecting the information about the rotation of, for example, an internal combustion engine.
2. Description of the Related Art
In a one known technique of detecting a change in magnetic field, electrodes are formed on either end of the magnetic field sensing plane of a magnetic field sensing element, and connection is made in such a manner as to form a bridge circuit, wherein the two opposite nodes of the bridge circuit are connected to a constant voltage source or a constant current source, whereby a change in the resistance of the magnetic sensing element is converted into a change in voltage, thereby detecting the change in the magnetic field.
FIG. 18 is a schematic diagram illustrating a conventional sensing device using a magnetoresistance (MR) element as the magnetic field sensing element, wherein FIG. 18A is a side view thereof and FIG. 18B is a perspective view.
This sensing device includes: a rotating shaft 1; a rotary magnetic material member 2 having at least one protruding or recessed portion and being adapted to rotate in synchronization with the rotation of the rotating shaft 1; a magnetoresistance element 3 disposed at a location a predetermined distance from the rotary magnetic material member 2; and a magnet 4 for applying a magnetic field to the magnetoresistance element 3, wherein the magnetoresistance element 3 includes magnetoresistance patterns 3a and 3b formed on a thin film plane (magnetic field sensing plane).
When the rotary magnetic material member 2 rotates, the magnetic field applied to the magnetic field sensing plane of the magnetoresistance element 3 changes. In response to the change in the magnetic field, the resistance of the magnetoresistance patterns 3a and 3b changes.
FIG. 19 is a block diagram illustrating a conventional sensing device using an MR element of the above-described type.
This sensing device includes: a Wheatstone bridge circuit 11 including magnetoresistance elements disposed a predetermined distance from the rotary magnetic material member 2 so that a magnetic field is applied from a magnet 4 to the magnetoresistance elements; a differential amplifier 12 for amplifying the output signal of the Wheatstone bridge circuit 11; a comparator 13 for comparing the output of the differential amplifier 12 with a reference value V.sub.TH and outputting a "0" signal or a "1" signal depending on the comparison result; a holding circuit 20 for holding the output of the comparator 13; and a waveform shaping circuit 14 for shaping the waveform of the output of the holding circuit 20 and supplying a "0" or "1" signal having sharply rising and falling edges to the output terminal 15.
The operation will be described below with reference to FIG. 20.
If the rotary magnetic material member 2 rotates, the magnetic field applied to the MR elements constituting the Wheatstone bridge circuit changes in response to the passage of the protruding and recessed portions of the rotary magnetic material member 2 as shown in FIG. 20A. As a result, the magnetic field sensing planes of the MR elements experience the change in the magnetic field corresponding to the protruding and recessed portions of the rotary magnetic material member 2. In response to the above change in the magnetic field, a change occurs in the mid-point voltage of the Wheatstone bridge circuit.
The difference between the mid-point voltages is amplified by the differential amplifier 12. Thus, as shown in FIG. 20B, the differential amplifier 12 outputs a signal corresponding to the passage of the protruding and recessed portions of the rotary magnetic material member 2 shown in FIG. 20A.
The output signal of the differential amplifier 12 is supplied to the comparator 13 which in turn compares the output signal of the differential amplifier 12 with the reference voltage V.sub.TH and outputs a "0" or "1" signal in response to the comparison result. After the output signal of the comparator is temporarily held by the holding circuit 20, it is shaped by the waveform shaping circuit 14 so that a "0" or "1" output signal having sharply rising and falling edges is obtained via the output terminal 15 as shown in FIG. 20C.
However, the conventional sensing device having the above construction has the following problems.
In the conventional sensing device, as shown in FIG. 21, a magnetic field is applied to a magnetoresistance pattern of a magnetic circuit in such a direction that when the magnet 4 faces a protruding portion of the rotary magnetic material member 2 as shown on the left of FIG. 21 (wherein N and S denote north and south poles of the magnet), the magnetic field emerging from the magnet 4 reaches the rotary magnetic material member 2 in a converging fashion after passing through magnetoresistance patterns 3a and 3b. In this case, both magnetoresistance patterns 3a and 3b have an equal resistance. When the magnet 4 faces a recessed portion of the rotary magnetic material member 2, although the magnetic field emerging from the magnet 4 reaches the rotary magnetic material member 2 in a diverging fashion, the magnetic field equally passes through the magnetoresistance patterns 3a and 3b, and therefore both the magnetoresistance patterns 3a and 3b have an equal resistance.
Thus, as shown in FIG. 21, no difference occurs in resistance between the magnetoresistance patterns 3a and 3b during the operation of detecting the change in the magnetic field corresponding to the protruding and recessed portions of the rotary magnetic material member 2. Therefore, as shown in FIG. 20, in the operation of detecting the protruding and recessed portions of the rotary member of magnetic material 2, the output of the differential amplifier 12 changes at the edges of the protruding and recessed portions wherein the output of the differential amplifier 12 has the same level for both the protruding and recessed portions. As a result, it is required to detect the edges and hold the detected signal by the holding circuit 20.
Another reason for the above problem is that the MR elements used in the conventional sensing device have no hysteresis in the characteristic of resistance versus applied magnetic field as shown in FIG. 22.
Since there in no difference in the output of the differential amplifier 12 between the protruding and recessed portions of the rotary member 2, it is impossible to obtain a signal exactly corresponding to the protruding and recessed portions of the rotary magnetic material member for a period just after the electric power to the sensing device is turned on (the ability of starting a precise operation immediately after the power is turned on will be referred to as "quick starting capability").
As described above, the problem of the conventional sensing device is that it is impossible to obtain a signal exactly corresponding to the protruding and recessed portions of the rotary magnetic material member. Another problem is that it is impossible to start a correct operation immediately after the electric power is turned on.
In view of the above, it is an object of the present invention to provide a sensing device capable of obtaining an output signal exactly corresponding to a predetermined position (angle), such as for a protruding or recessed portion of a rotary magnetic material member, and also having the quick starting capability.